Understanding the rheological properties of deformable polymer particle suspension in the melt state

In this work, we study the rheology of suspensions of polystyrene (PS) spheres at concentrations ranging from 8 to 66% (v/v) in a matrix of molten ethylene vinyl acetate copolymer (EVA) at temperatures spanning through the glass transition temperature of the PS spheres. As a result of the significant difference between the softening temperatures of these blend components, the melt order plays a crucial role in the generation of the blend morphology, rheology, and optimization of processing conditions. When the PS is rigid (below \({T}_{g}\) $T_g$ ), the suspensions showed significant Mullins and Payne effects, disappearing at higher temperatures where PS was sufficiently softened. The origin of these effects is postulated to be due to the extensive filler-filler contacts and radial alignment of the PS beads. Near the glass transition of PS, the PS spheres became deformable and viscosity of the suspension showed an unusual variation as a function of PS loading. The Krieger-Dougherty model was used to predict viscosity variation with volume fraction. We demonstrate that the deformability of the “solid” PS spheres significantly affects the reduced suspension viscosity at both intermediate volume fractions of 0.42 to 0.44, where there was an anomalous dip in viscosity, and at higher volume fractions beyond 0.60, where the increase in viscosity was not as prounounced as that for suspensions of rigid particles.